Certain alloys of nickel and cobalt may exist in an amorphous form. They contain nickel, cobalt and/or iron and specified proportions of such elements as molybdenum and/or tungsten, and boron, silicon and/or carbon. The alloys are prepared with the amorphous structure by rapid quenching from the melt. For example amorphous ribbon may be produced by quenching a stream of molten alloy on a chilled surface as described in U.S. Pat. No. 4,116,682. A practical method of processing such alloys into a directly useful form is by thermal spraying to produce a coating.
Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A conventional thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 2 microns. A thermal spray gun normally utilizes a combustion or plasma flame to produce the heat for melting of the powder particles. It is recognized by those of skill in the art, however, that other heating means may be used as well, such as electric arcs, resistancme heaters or induction heaters, and these may be used alone or in combination with other forms of heaters. In a powder-type combustion thermal spray gun, the carrier gas, which entrains and transports the powder, can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air. In a plasma spray gun, the primary plasma gas is generally nitrogen or argon. Hydrogen or helium is usually added to the primary gas. The carrier gas is generally the same as the primary plasma gas, although other gases, such as hydrocarbons, may be used in certain situations.
The material alternatively may be fed into a heating zone in the form of a rod or wire. In the wire type thermal spray gun, the rod or wire of the material to be sprayed is fed into the heating zone formed by a flame of some type, such as a combustion flame, where it is melted or at least heat-softened and atomized, usually by blast gas, and thence propelled in finely divided form onto the surface to be coated. In an arc wire gun two wires are melted in an electric arc struck between the wire ends, and the molten metal is atomized by compressed gas, usually air, and sprayed to a workpiece to be coated. The rod or wire may be conventionally formed as by drawing, or may be formed by sintering together a powder, or by bonding together the powder by means of an organic binder or other suitable binder which disintegrates in the heat of the heating zone, thereby releasing the powder to be sprayed in finely divided form.
A class of materials known as self-fluxing alloys are quite common for hard facing coatings produced by such methods as thermal spraying. These alloys of nickel or cobalt contain boron and silicon which act as fluxing agents during processing and hardening agents in the coating. Usually self-fluxing alloys are applied in two steps, vis. thermal sprayed in the normal manner and then fused in situ with an oxyacetylene torch, induction coil, furnace or the like, the fluxing agents making the fusing step practical in open air. However, the alloys may also be thermal sprayed with a process such as plasma spraying without requiring the fusing step, but the coatings are not quite as dense or wear resistant. Generally self-fluxing alloy coatings are used for hard surfacing to provide wear resistance, particularly where a good surface finish is required.
A typical self-fluxing alloy composition of nickel or cobalt contains chromium, boron, silicon and carbon. An alloy may additionally contain molybdenum, tungsten and/or iron. For example U.S. Pat. No. 2,868,639 discloses an alloy for hard surface composed of (by weight) 7 to 17% chromium, 1 to 4.5% boron, 1 to 5.5% silicon, 0.1 to 5.5% iron, 6 to 20% of at least one of tungsten and molybdenum, 0.05 to 2.5% carbon, the remainder nickel and incidental impurities. U.S. Pat. No. 2,936,229 discloses a cobalt alloy containing 1.5 to 4% boron, 0 to 4% silicon, 0 to 3% carbon, 0 to 20% tungsten and 0 to 8% molybdenum.
U.S. Pat. No. 2,875,043 claims a spray-weldable alloy containing at least 40% nickel, 1 to 6% boron, silicon up to about 6%, 3 to 8% copper and 3 to 10% molybdenum. Tungsten is not included.
Some of the self-fluxing alloys have been in use commercially for more than 25 years and have been quite successful. These alloys have melting ranges around 1075 degrees Centigrade and hot hardness is lost at a temperature as low as 650 degrees; therefore self-fluxing alloys are not useful at high temperature. Also, if very high wear resistance is needed a carbide such as tungsten carbide is added as described, for example, in British Patent Specification No. 867,455. Carbides are expensive and make the coatings difficult to grind finish, harder to fuse and less resistant to corrosion.
European Patent Specification No. 0 009 881 (published Jan. 11, 1984) involves an alloy composition of at least 48% cobalt, nickel and (if present) iron; 27 to 35% chromium; 5 to 15% molybdenum and/or tungsten; 0.3 to 2.25% carbon and/or boron; 0 to 3% silicon and/or manganese; 0 to 5% titanium and the like; 0 to 5% copper; and 0 to 2% rare earths. There are, however, certain restrictions including that if there is 2% or more of carbon and/or boron present, there is more than 30% chromium present.
More than 10% iron is preferred. Also, preferably no boron is present or, if it is present, it should not constitute more than 1% of the composition; and further limitations on boron are indicated where a significant amount of carbon is present.
U.S. Pat. No. 4,116,682 describes a class of amorphous metal alloys of the formula MaTbXc wherein M may be iron, cobalt, nickel and/or chromium, T may include molybdenum and tungsten and X may include boron and carbon. The latter group X of boron, etc. has a maximum of 10 atomic percent which calculates to about 1.9% by weight maximum for boron in the amorphous alloys; thus boron is characteristically low compared to the boron content in self-fluxing type of alloys, although there is some overlap. One typical amorphous composition is (by atomic percent) 58 nickel, 25 chromium, 2 iron, 5 molybdenum, 3 tungsten, 4 boron, 3 carbon. As weight percent this is (approximately) 22% chromium, 1.8% iron, 8% molybdenum, 10% tungsten, 0.7 boron, 0.7 carbon, balance nickel.
The amorphous types of compositions are of growing interest for the combined properties of corrosion resistance, frictional wear resistance and abrasive wear resistance. However, further improvements in these properties are desired.
In view of the foregoing, a primary object of the present invention is to provide a novel alloy composition characterized by the combination of corrosion resistance, frictional wear resistance and abrasive wear resistance.
A further object of this invention is to provide an improved amorphous type of alloy for the thermal spray process.
Another object is to provide an improved thermal spray process for producing corrosion and wear resistant coatings.